Japanese Patent Application Publication No. 2003-15477 describes a technique for estimating the number of days remaining until a printer can no longer print. Specifically, the printer measures the residual quantity of developer remaining in a cartridge every three days, and stores, in a memory, the residual quantity together with the number of days in which the cartridge accommodating the developer has been in use. Subsequently, the printer uses the plurality of residual quantities already stored in the memory to calculate a graph line for predicting future residual quantities of the developer. The printer then specifies a date corresponding to the residual quantity of the developer that is insufficient for printing on the graph line as an estimated date at which printing will no longer be possible. The graph line is a straight line that passes through the residual quantity on the date when the cartridge has begun to be used and a point specifying the most recent residual quantity.